As for the event itself:
The event starts on the 19th, and we are getting it for a really long time as it goes to the 10th of May - 3 full weeks. It is naturally a huge campaign like KnK/Saber Wars, not a tiny one like Valentines or the present one.
Here's how it works:
Main Quest
The main quest contains 18 acts and an act EX. Like the KnK even, these main quests will become available by doing missions. Act 18 rewards the player with a new servant - Irisviel, a decent caster.
Missions
Like the KnK event, we have goals - missions. For instance, "Defeat 5 Hassan (Enemy)" which'll give 5 void dust as a reward and unlocks a free quest.
Free Quests
There are a bunch of free quests to grind, and each one has 5 levels to them - D, C, B, A, and EX. D and C will be the only ones available at the start. The do B and A, you need an event currency - of which there are 4. We have Sea Demon's foots, golden chalices, assassin's masks, and magic ore fragments. EX requires clearing act 18 of the main quest - which is near the end of the grind in the first place. The EX quests unlock the Crystal Balls - Irisviel's ascension item.
Event currency
There are 3 ways to get this event currency. 10 of each are bought with "insignias" - more on that later. You also get 6 of the currency through specific missions, and 4 of the currency by doing main quests. Obtaining these unlocks B and A level events - B requires anywhere from 4 to 13 of an item, A anywhere from 14 to 17.
Insignias
Killing a mob will reward you with its insignias. So lancer drops lancer insignias, much like Valentines day. There are 3 ways to boost these. First, you can bring in an event servant - which is any form of Gil, Hassan, Lancelot, Alexander, Waver, Diarmuid, and Gille de Rais. Only vanilla Artoria Pendragon will count. There is also an Emiya (Assassin) who will count in this event. Using these classes in a class advantage situation (IE, Artoria vs. Lancers) boosts drops by 3. Lancelot will boost Berserker insignias by 3, and no others. There is also the FP servant, Angra Mainyu, who will boost drops of all classes by 3, if you're lucky enough to get him.
With Rider/Caster/Assassin/Berserker insignias you can buy the "Final Destination" craft essence, which boosts drops by 2 for all classes. You will get one of these through missions as well.
For those paying you can also get craft essences through the summoning pool to increase drops of the equipped classes by 1/2/3, but these don't seem as useful as the Final Destinations.
And of course, there are other goodies in the currency shops - Fous, Experience Cards, Mana Prisms, Eternal Wheels, Horns, Pages, Spirit Roots, Octuplet Crystals, Dragon Scales, and all classes monuments and QP.
Bonus Quests
After clearing the last main mission - Act EX - and collecting 20 of each event currency, you will gain access to 4 new quests. Each one has 3 stages only requiring completion of the last stage to do. Clearing all 3 will reward another Irisviel to allow you to NP5 her.
When you clear those, you should have all missions done. You can then do the last bonus quest which will have you facing the final boss. Reward is 5 Black Beast Greases (which for me is the jackpot~)
Conclusion
For me, I'm planning on going ham on this event. It's a huge fun event, and honestly it'll be a while until we get one like this again. In Japan after this there was a 2 week break until Ibaraki Douji, which is really just one quest done as a raid. After that is Journey to the West which is sorta like the current campaign. Then we have 2 weeks until Onigashima, which is a bit more complicated. Then Camelot, and then finally, the summer event - which to me is the next huge event. ^^
